This invention relates to Synchronization of oscillators, and more particularly to synchronization of oscillators of modems at customer premises equipment (CPE) to oscillators at the Local Exchange carrier (LEC) in a Discrete Multi-Tone (DMT) based digital subscriber loop (DSL).
For purposes of the digital subscriber loop, the clock at the LEC is considered to be the master clock, and the CPE modem needs to synchronize its sampling clock to that of the LEC. Modems that are based on DMT technology are governed by the T1.413 ANSI Standard, which specifies a frame that contains a Pseudo-Random Number sequence (PRN). The PRN signal frame is used during acquisition and steady state data transmission. During the acquisition period numerous PRN frames are sent back and forth between the modems at the LEC that contains the master clock and the CPE modem that contains the slave clock. These frames are used for channel estimation and for acquisition of frame synchronization. According to the Standard, after frame synchronization is established, the standard nevertheless calls for a PRN frame to be injected into the signal stream every 69 frames. Conventionally, the injected PRN frame is used to maintain frame synchronization.
To maintain clock synchronization, the ANSI Standard specifies one frequency bin in the DMT schema that is to be used as a pilot signal. This, of course, reduces the overall throughput of the system, because one frequency bin is made unavailable by the pilot signal.
An advance in the art is realized by refraining from using the provided-for frequency bin for the pilot signal and, instead, using this bin for data. Oscillator synchronization is achieved via control signals that are developed from the PRN frame that arrives once every 69 frames. The control signal is developed by rotating the vectors derived from the FFT-demodulated received signal, applying a complex tracking filter to the results, averaging the results, performing phase accumulation, and developing an analog voltage to be applied to a voltage controlled oscillator through D/A conversion.